This invention describes the preparation of improved polyimide gas separation membranes and gas separation processes that utilize these membranes. Specifically, soluble polyamic acid salt (PAAS) precursors comprised of tertiary and quaternary amines, ammonium cations, sulfonium cations, or phosphonium cations, are prepared and fabricated into membranes that are subsequently imidized and converted into rigid-rod polyimide membranes with desirable gas separation properties. A method of tailoring membrane gas separation and permeation characteristics by controlling the imidization process and PAAS counter ion selection without changing the chemical structure of the polyimide membrane is also disclosed.
The use of polymeric membranes for gas separation applications is well documented in the art. The relationship between the polymeric structure and the gas separation properties has been extensively studied, see for example, W. J. Koros, Journal of Membrane Science, Volume 83, pp1, 1993; L. M. Robeson, Journal of Membrane Science, Volume 62, pp165, 1991; and L. M. Robeson, Polymer, Volume 35, pp4970, 1994. It is well documented in the art that stiffening the polymeric backbone while simultaneously inhibiting chain packing can lead to improved gas permeability combined with an increase in gas selectivity for certain gas mixtures. Polyimides are examples of such rigid-rod polymers showing desirable gas separation properties, see for example, D. R. B. Walker and W. J. Koros, Journal of Membrane Science, Volume 55, page 99, 1991; S. A. Stern, Journal of Membrane Science, Volume 94, page 1, 1994; K. Matsumoto, P. Xu, Journal of Applied Polymer Science, Volume 47, page 1961, 1993,). U.S. Pat. Nos. 4,705,540; 4,717,393; 4,717,394; 5,042,993; and 5,074,891 disclose the preparation of such aromatic polyimide gas separation membranes. For practical industrial applications polymeric gas separation membranes are fabricated into an asymmetric or a composite configuration with thin separation layers. The membranes can be further configured into flat sheets or into hollow fibers. Although rigid-rod polyimides have excellent gas separation properties, they frequently can be dissolved only in aggressive organic solvents such as N-methyl-pyrrolidinone (NMP), N,N-dimethyl formamide (DMF), or phenols which makes it difficult to prepare composite membranes with ultrathin separation layers and can further cause environmental problems. For example, polyimide membranes have been fabricated from chlorophenol solutions as described in U.S. Pat. No. 4,440,643.
U.S. Pat. Nos. 5,618,334; 5,725,633; and 5,744,575 disclose modified polyimides containing sulfonic acid groups that exhibit improved solubility in common organic solvents. U.S. Pat. Nos. 4,440,643 and 5,141,642 disclose the process of fabricating polyimide gas separation membranes from polyamic acid precursors. However, polyamic acids can undergo degradation and are sensitive to temperature and moisture variations, which makes the manufacturing of polyamic acid membranes that exhibit reproducible properties most difficult. Furthermore, some polyamic acids are not soluble in mild organic solvents, and all polyamic acids require harsh conditions to complete imidization. For example, temperatures as high as 300xc2x0 C. are generally required to complete imidization of polyamic acids by thermal treatment. The limited availability of solvent systems and high imidization temperatures prohibits the application of polyamic acid precursors as the coating material for the fabrication of composite polyimide membranes when preferred, readily available polymeric substrates, such as polysulfone are used. To maintain the high porosity of the porous substrate the thermal imidization temperature must be lower than the glass transition temperature (Tg) of the porous substrate polymer. Most of the commercially employed polymeric substrates have glass transition temperatures below 200xc2x0 C., for example, the Tg of polysulfone is 190xc2x0 C.
It is known in the art that polyimide polymers can be prepared from polyamic acid salt precursors. U.S. Pat. Nos. 4,290,929 and 5,719,253 disclose the use of polyamic acid solutions of tertiary amine. The following publications also disclose the synthesis of polyamic acid salts: R. J. W. Reynolds and J. D. Seddon, Journal of Polymer Science, Part C, Volume 23, pp45, 1968; and J. A. Kreuz, A. L. Endrey, F. P. Gay, and C. E. Sroog, Journal of Polymer Science, Part A-1, Volume 4, pp 2607, 1966; Y. Echigo, N. Miki, and I. Tomioka, Journal of Polymer Science, Polymer Chemistry, Volume 35, pp2493, 1997.
It has been taught in the art that amphiphilic polyamic alkylamine salts can form Langmuir-Blodgett (LB) films on water surfaces that subsequently can be converted into polyimide films, see, for example, U.S. Pat. No. 4,939,214 as well as Y. Nishikata, et al., Polymer Journal, Volume 20, pp269, 1988, and Y. Nishikata, et al., Thin Solid Films, Volume 160, pp15, 1988. Marek et al. disclose preparation of thin LB films for gas separation applications from dimethyldodecyl-ammonium and dimethylhexadecyl-ammonium polyamic acid salts, see M. Marek et al., Polymer, Volume 37, pp2577, 1996. The authors concluded that LB films with gas separation characteristics cannot be obtained from the short-chain tertiary amine salts of polyamic acid. Marek et al. found that to form LB films that exhibit gas separation property, one of the alkyl chains in the tertiary amine salt has to be longer than 16 carbon atoms to form an acceptable LB film. The use of polyamic acid salt precursors for membrane preparation in the prior art is thus limited to LB film techniques which are not practical for commercial membrane production due to the requirement of forming films on a water surface, then transfering such film to a porous substrate. Therefore, a need still remains for improved methods to prepare polyimide membranes, in particular methods that employ mild organic solvents and/or mild heat or chemical treatments in polyimide membrane preparation.
The present invention teaches an improved and industrially feasible method to fabricate polyimide gas separation membranes. In a preferred embodiment, the polyimide membranes of the present invention are produced by a two-step process, wherein: (a) a membrane is formed from a polyamic acid salt membrane precursor that contains the following units in its structure: 
wherein R is a substituted or unsubstituted aromatic, alicyclic, heterocyclic, or aliphatic radical;
X is an ammonium ion, a phosphonium ion, a sulfonium ion, a protonated tertiary amine, a quaternary amine or mixtures thereof; the quaternary amine ion can be a heterocyclic, alicyclic or an aromatic amine ion or an ion of the following general formula: R1R2R3R4N+; the protonated tertiary amine can be a heterocyclic amine, alicyclic amine, an aromatic amine or an amine of the following general formula: R1R2R3NH R1, R2, R3 and R4 can be the same or different and are aryl or alkyl radials; and (b) the membrane formed from the polyamic acid salt precursor is subsequently converted into the polyimide membrane by thermal or chemical treatment.
The invention is based on the unexpected findings that certain polyamic acid salts, preferably tertiary amine salts or quaternary salts of polyamic acids, have very good solubility in mild organic solvents and that the imidization of these polyamic acid salts can be realized at relatively mild thermal conditions. These characteristics are extremely useful for the fabrication of gas separation membranes, in particular, composite membranes.
In one embodiment of this invention, the polyimide gas separation membrane is produced by (a) preparing a membrane casting solution containing polyamic acid salt in at least one polar solvent; (b) forming a membrane configuration shaped as a flat sheet or as a hollow fiber from the polyamic acid salt solution; (c) conveying said membrane configuration through an evaporation zone; (d) bringing said membrane configuration into contact with a coagulating liquid to form a solidified membrane; (e) washing said solidified membrane to remove residual solvent; and (f) drying the solid membrane. (h) converting the polyamic acid salt membrane into a polyimide membrane by heat or chemical treatment. In a preferred embodiment, a caulking procedure may be further applied to the PAAS membrane surface, and/or to the surface of the polyimide membrane to repair defects and imperfections.
In another embodiment of the invention, a polyimide gas separation membrane is produced by (a) forming a solution of polyamic acid salt in a solvent system; (b) applying the thus formed solution to a porous substrate to form a coating layer; (c) solidifying the coating layer by drying or by immersing it into a nonsolvent followed by drying; and (d); converting the polyamic acid salt layer into a polyimide layer In a preferred embodiment, a caulking procedure may be applied to the membrane surface, and/or to the surface of the polyimide membrane to repair defects.
In a further embodiment of this invention, a porous polyimide membrane is formed by forming a porous polyamic acid salt membrane that is then converted into a porous polyimide membrane.
In an even further embodiment, the gas permeation characteristics of the polyimide membranes prepared according to the teaching of the present invention can be tailored by changing the counter ions in the PAAS precursors and by controlling the imidization process conditions. This feature allows one skilled in the art to modify the gas separation and permeation characteristics of polyimide membranes without utilizing the conventional method of modifying the chemical structures of the polyimides, an approach that is limited by the availability of monomers and the economics of the polymer synthesis.
Other features and advantages of the present invention will become apparent from the following description of the invention.